1. Field of the Invention
This invention relates to a process for preparing random copolymer functionalized at both terminals and more particularly, to the process for preparing a conjugated diene-aromatic vinyl random copolymer having functional groups at both terminals using a difunctional initiator derived from a vinyl disubstituted aromatic material so that it has an excellent combination of properties such as improved rolling resistance and improved wet skid resistance for use in making tire treads.
2. Description of the Prior Art
Rubber materials for the use of tire treads in automobile should have lower rolling resistance in terms of fuel-saving purpose and higher wet skid resistance for the safety of drivers.
According to a I. R. C. report (Kyoto, 1985) released by T. Fujimaki and M. Ogawa in Bridgestone, it was reported that the rolling resistance of tire was responsible for the total energy loss in automobile by 14.4% and the only 10% reduction of rolling resistance only might contribute to saving the automobile fuel by 1-2%.
In order to reduce the rolling resistance of tire, however, a rubber material should have a very high elasticity. Also, to improve the tire traction against wet roads affecting the safety of driver, a very high wet skid resistance should be ensured. In this context, various methods have been disclosed so as to prepare a rubber which has a balanced combination of the two properties.
The first method is to control the content of a monomer and its fine structure so as to improve the physical properties, as shown in the following: According to the U.S. Pat. Nos. 4,843,120, 5,047,483 and 5,159,020 including the European Pat. Nos. 0,349,472 and 0,438,966, a method for synthesizing a rubber prepared in such a manner that conjugated diene segments having different fine structures are contained in a polymer chain so as to improve a tire traction property, has been disclosed. In addition, the U.S. Pat. No. 5,612,436 has disclosed a method of improving the traction characteristic of tire treads using an isoprene-butadiene diblock random copolymer without sacrifice of rolling resistance.
The second method is to introduce an initiator containing functional groups to a polymer, as shown below.
The European Pat. No. 0,476,640 has disclosed a method for preparing an functionalized initiator by reacting a conjugated diene and a trimetalated alkyne which in turn is prepared from the reaction between 1 equivalent of 1-alkyne and 3 equivalent of organic metal. In spite of the fact that this method can introduce functional groups at the terminal of polymer, a longer reaction time may result in reducing the activity of initiator with poor solubility. The European Pat. No. 0,626,278 has disclosed a method for utilizing an amino lithium as an initiator and the U.S. Pat. No. 5,523,371 has disclosed a method of utilizing adducts of amino-substituted aromatic materials and alkyl lithiums as initiators. However, this methods are unsuitable for industrialization in the aspect of solubility and control of activity.
The third method is to add a coupling agent or functionalizing material to a living copolymer. In particular, the U.S. Pat. Nos. 4,540,744, 5,541,264 and 4,519,431 have disclosed a method of coupling the terminal of polymer with SnCl4 so as to effectively reduce the hysteresis. Among other things, a Sn--C bond is easily decomposed and its resultant radical is easily reacted with functional groups at the surface of carbon black, thus preventing the formation of network derived from carbon particles with the formation of a polymer-carbon black bond which may result in further improvement of physical properties and processibility.
Also, several methods have been disclosed on the improvements of physical properties through end-capping of living polymer using various functionalizing materials as follows: The European Pat. No. 0,510,410 has disclosed a method of adding 0.3-4 equivalents of a substituted imine material to a living chain of diene polymer or copolymer; The U.S. Pat. No. 5,317,057 has disclosed a method of adding 0.8-1.5 equivalents of halo methyl vinyl arene to a living polymer chain; The U.S. Pat. No. 5,346,962 has disclosed a method of introducing a vinyl substituted imidazole into an terminal of polymer chain.
In particular, the U.S. Pat. No. 5,281,671 has disclosed a method for preparing a polymer which has improved breaking strength or friction resistance as well as improved rolling resistance and wet skid resistance, through introduction of N-substituted material to an active terminal of polybutadiene having 70-95 wt % of trans-1, 4-bond and blending with other conjugated diene rubbers.
The U.S. Pat. No. 4,616,069 and the European Pat. No. 0,180,141 have disclosed a method in which a conjugated diene copolymer is prepared using an organo-metallic initiator in the presence of a conjugated diene monomer or aromatic vinyl monomer and hydrocarbon solvent, while some parts of such copolymer are coupled with SnCl.sub.4 and other remaining parts are reacted with amino aldehyde or amino ketone, thus improving a rolling resistance.
The U.S. Pat. Nos. 4,614,771 and 3,755,269 including the European Pat. No. 0,180,853 have disclosed a method for a diene copolymer rubber having improved rebounding property in such manner that a conjugated diene-aromatic vinyl copolymer, so prepared, is reacted with amino aldehyde or amino ketone. The material functionalized at the terminal has indicated improved physical properties and processibility for use as a tire tread rubber (ref.: Rubber Chemistry & Technology, vol. 60, pp. 837-855).
Nevertheless, all of these methods are designed to improve tire treads by introducing a functionalized material at one terminal of copolymer, to promote an interaction with a filler.
Meantime, the conventional methods for the preparation of difunctional initiator are as follows:
According to the U.S. Pat. Nos. 4,196,154, 4,201,729, 4,200,718, 4,205,016, 4,172,190 and 4,196,153 have disclosed a multifunctional lithium-containing initiator dissolved in hydrocarbon solvent and its application to the preparation of a block copolymer; The European Pat. No. 0,413,294 and the U.S. Pat. No. 5,750,055 have disclosed a method for preparing a block copolymer in such a manner that a small amount of conjugated diene is added to a difunctional initiator for preparing .alpha.,.omega.-dilithio poly(conjugated diene) solution. Techniques disclosed in these patents involved complicated reaction conditions in order to prepare a block copolymer having a vinyl content of less than 10% and small molecular weight distribution.
Further, a process for preparing a block copolymer containing 1,3-diisopropenylbenzene has been suggested (ref.: Macromolecules 1996 (vol. 29) pp. 2738-2745, 1994 (vol. 27) pp. 5957-5963, 1997 (vol. 30) pp. 4254-4261, 1997(vol. 30) pp7356-7362, 1996 (vol. 29) pp. 1753-1761). This process has suggested some laboratory conditions designed to exhibit comparable physical properties of a block copolymer such as tensile strength and elongation, which are realized through coupling or 3-step polymerization, but further improvement should still be made in terms of reaction conditions or productivity.